Low-speed Wind-tunnel Investigation Research Articles

Experimental Study of an Asymmetric Tandem Wing Configuration for Drone Application

A low-speed wind tunnel investigation characterizing the performance of an asymmetric tandem wing is presented. Wind tunnel data were supported by numerical modeling and flow visualization. The opposing wing halves of a rectangular AR=6.23 wing were systematically shifted fore and aft along a central fuselage to yield asymmetry. The results show that the lift curve slope, minimum drag coefficient, and Oswald efficiency factor are weakly affected by asymmetry, a result supported by numerical computations. The maximum lift-to-drag ratio, however, dropped essentially linearly with increasing asymmetry, a consequence of the reduced aspect ratio of each wing half decreasing the lift coefficient for minimum drag. Comparison of the experimental results with a symmetric wing with conventional empennage suggests that an asymmetric tandem wing configuration may be a viable geometry for deployable flight vehicles such as tube-launched drones.

Efficient Low-Reynolds-Number Airfoils

A low speed wind tunnel investigation is presented characterizing the effect of leading and trailing edge flaps on thin flat plate airfoils at low Reynolds numbers. Flap size and setting angle were varied. Tests were conducted at Reynolds numbers of 40,000, 60,000, and 80,000, consistent with the operational regime of micro aerial vehicles. Performance was contrasted against a circular arc profile and three conventional airfoils. Testing encompassed force balance and surface pressure measurement. The numeric data were complemented by on and off surface flow visualization as well as the development of an analytic prediction model. The results indicate that the deployment of leading and trailing edge flaps can significantly improve the airfoil’s efficiency—not just compared with a flat plate but also a 5% camber circular arc profile. For the tested geometries, a leading and trailing edge flap angle of 15 deg was optimal. Conventional airfoil performance degraded significantly at low Reynolds numbers, yielding efficiencies far below any of the flat plate airfoils for and 60,000. Increasing the Reynolds number to 80,000 showed a dramatic improvement in the performance of the conventional airfoils.

Experimental study of drag reduction on a golf driver club using discrete surface elements

A low-speed wind tunnel investigation is presented documenting drag reduction to a golf driver club. Geometrical modifications (or elements) were attached to the golf driver. The size, spacing, and location of the elements were varied. Wind tunnel velocities spanned a range from the amateur to professional golfer. Measurements included both force balance and surface pressure accompanied by flow visualization to aid in flow diagnostics. The results indicated that a 40% reduction in drag can be achieved, primarily due to reduced pressure over the forward face of the crown and increased pressure over the crown’s aft extents. This reduction in drag was estimated to yield a 0.54 m/s (1.2 mph) club head speed gain, causing a 3.65 m (4 yds) increase in carry distance.

Effects of Plain and Gurney Flaps on a Nonslender Delta Wing

A low-speed wind tunnel investigation was conducted to characterize the performance of plain and Gurney flaps on a 55 deg sweep delta wing. The geometry of the plain trailing-edge flaps was varied in terms of their length and setting angle. The height of the Gurney flaps was altered. Both the trailing-edge flaps and the Gurney flap are effective at lift augmentation; however, the plain flap has only a moderate associated drag penalty, while that of the Gurney flap is large. The drag increment of the Gurney flap is proportional to its relative area compared to that of the wing and far outweighs the Gurney flap’s associated lift increment. Consequently, this leads to a significant drop in wing efficiency compared to the trailing-edge flap or planar wing. The addition of either type of flap causes a pitching moment reversal. Analytic modeling of both flaps successfully captured the aerodynamic effect of the flaps as a function of their geometric variables.

Investigation of a Morphable Parawing for Unmanned Aerial Vehicle Application

A low-speed wind-tunnel investigation is presented characterizing the performance of dual-lobed parawings. Parawings are conically shaped geometries that possess significant washout and camber variation across the span. A series of models with a planar leading-edge sweep of 55 deg increasing to 65 deg to form the parawing canopy were tested. Asymmetric wing tests were also conducted to explore the ability of the configurations to generate moments suitable for roll control. The results indicate a significant reduction in lift with increasing wing slackness stemming from both a reduction in the lift curve slope and positive shift in the zero-lift angle of attack. The angle of attack for zero lift is shown to correlate strongly with the peak lobe height. Asymmetric wing setting showed the generation of rolling moments comparable in magnitude with those produced by traditional ailerons. Numerical simulation indicated that a freely available lifting-surface theory application is able to accurately simulate the performance of the parawing geometry.

Effect of Gurney Flaps on an Elliptical Airfoil

A low-speed wind tunnel investigation is presented characterizing the impact of Gurney flaps on an elliptical airfoil. The chordwise attachment location and height of the flaps were varied, as was the Reynolds number. The results showed strong nonlinearities in the lift curve which were present for all tested geometries. Flap effectiveness was seen to diminish as the flap was moved closer to the trailing edge stemming from flap submersion in separated flow. For the tested cases, the measured lift coefficients showed a weak Re dependency. The upper airfoil surface was shown to carry approximately 80% of the total lift load. The top surface caused a pitching moment reversal associated with nonlinearity in the lift curve.

Experimental Investigation of the Effect of Trailing-Edge Flaps on Delta Wings

A low-speed wind-tunnel investigation is presented, characterizing the effect of plain trailing-edge flaps on 65 and 70 deg sweep delta wings. Four flap angles and three hinge locations were evaluated. The leading-edge suction analogy was extended to allow estimation of the effect of flaps on both the attached flow and vortex lift components. Additionally, expressions to estimate the drag and pitching moment coefficient were also developed. Comparisons of theory and experiment showed close accord. Flap deflection was observed to cause a notable reduction in the drag coefficient for a given lift coefficient. Wing stall, however, was observed at lower angles of attack than without the flap, caused by an earlier onset of vortex breakdown.

Effect of Leading-Edge Slats at Low Reynolds Numbers

One of the most commonly implemented devices for stall control on wings and airfoils is a leading-edge slat. While functioning of slats at high Reynolds number is well documented, this is not the case at the low Reynolds numbers common for small unmanned aerial vehicles. Consequently, a low-speed wind tunnel investigation was undertaken to elucidate the performance of a slat at Re = 250,000. Force balance measurements accompanied by surface flow visualization images are presented. The slat extension and rotation was varied and documented. The results indicate that for small slat extensions, slat rotation is deleterious to performance, but is required for larger slat extensions for effective lift augmentation. Deployment of the slat was accompanied by a significant drag penalty due to premature localized flow separation.

Effect of a pusher propeller on a delta wing

A low speed wind tunnel investigation was conducted to examine the effect of a pusher propeller on a 65 deg sweep delta wing. Two different propeller diameters were evaluated. The data showed that the pusher geometry can cause a delay in vortex breakdown yielding an increase in lift at high angles of attack. Surface pressure measurement showed that the propeller increased suction levels on both the leeward and windward surfaces, limiting lift enhancement. Lift augmentation was observed to favor low advance ratios.

Effect of Taper Ratio at Low Reynolds Number

A low speed wind-tunnel investigation is presented documenting the impact of taper ratio at low Reynolds number. An wing using a S8036 section was tested at and 150,000. Taper ratio was varied from 1 down to 0.2 in 0.2 increments. An analytic prediction method as well as extended numerical lifting line theory incorporating nonlinear section data was implemented to compare with experiment. The experimental results indicated a weak dependence of peak lift to drag ratio on taper ratio for both Reynolds numbers. Surface flow visualization showed the presence of a large laminar transitional bubble on the upper wing surface. The relative extent of the bubble was observed to increase towards the tip region for tapered planforms. Agreement between theory and experiment in terms of lift and drag estimation was good. Planform optimization using the extended numerical lifting line theory indicated that at the test conditions and using the selected airfoil section, a planform that differs from straight taper or elliptic may be most efficient.

Experimental and Analytic Investigation of Ground Effect

A low-speed wind-tunnel investigation was conducted to examine the effect of aspect ratio on ground effect. The characteristics of the wing and its section (a S8036 profile) were established at . The results indicate that ground effect is highly beneficial in augmenting lift, such that marked increases in the maximum lift-to-drag ratio may be realized. Finite wings in ground effect benefit from a reduction in vortex drag, as well as a lift increase, yielding greater gains in efficiency than achievable for an airfoil. The evaluation of analytic relations for estimation of the finite wing lift-curve slope and drag due to lift showed close accord with experiment.

Effect of Aspect Ratio on Gurney-Flap Performance

A low-speed wind-tunnel investigation has been undertaken to establish the effect of wing aspect ratio on Gurney-flap performance. Characterization is accomplished using a force balance and flow visualization. The Gurney-flap lift increment due to a shift in the zero-lift angle of attack was observed to scale with that of the lift-curve slope for different aspect ratios. As the aspect ratio reduced, a Gurney flap of greater height was required to maximize aerodynamic efficiency. The dependence of aerodynamic parameters (zero-lift angle of attack, minimum drag coefficient, and lift-curve slope) on the Gurney flap’s height-to-chord ratio was also examined.

Effects of Sweep and Nonplanarity on a Delta-Winged Biplane

A low-speed wind-tunnel investigation is presented, exploring the effect of delta wings configured in an 'X' biplane geometry. Wing leading-edge sweeps ranging from 50 to 70 deg and anhedrat/dihedrai angles up to 40 deg were explored. Results indicated a significant attenuation of vortex lift production, while the attached now lift component was enhanced due to nonplanarity, an outcome supported by theory developed to analytically simulate this wing geometry. Observed benefits of this wing configuration amplify with wing slenderness, where marked increases in the range parameter may be realized. Both theory and experiment indicate that the optimal anhedral/dihedral angles for the tested geometries ranged from 20 to 30 deg.

Experimental Investigation of a Morphable Biplane

A low-speed wind-tunnel investigation was conducted to evaluate the behavior of a morphable biplane configuration.Morphing encompassed a rotation of the wings around the fuselage such that they could bemerged to form amonoplane or split to form anX-shaped biplane. Circular arc and Clark Y wing sections were evaluated. The Reynolds number as well as the wing’s anhedral/dihedral angle were varied. The results indicated that the biplane increased the lift curve slope,maximum lift, and endurance parameter.However, the increase inwetted area caused a reduction in the range parameter. The proposed morphing concept would be beneficial for low-aspect-ratio geometries.

Airfoil Lift Augmentation at Low Reynolds Number

A low-speed wind-tunnel investigation was undertaken to explore the behavior of trailing-edge flow effectors on the performance of a S8036 airfoil. Gurney flaps, as well as conventional trailing-edge flaps, were explored at Re = 40, 000,60,000, and 80,000. The effects of Gurney flap length, as well as trailing-edge flap length and deflection angle, were studied. Performance characterization encompassed force balance and pressure measurements. Examination of the results showed that both types of flaps are capable of significantly increasing not only the maximum lift coefficient, but also the range and endurance parameters. For the geometries explored, both the Gurney and trailing-edge flap showed similar levels of lift augmentation; however, the trailing-edge flaps generally showed larger relative increments in the lift-to-drag ratio and endurance parameter than the Gurney flaps.

Low-Reynolds-Number Airfoil Investigation of Lower-Surface Leading-Edge Flaps

A low-speed wind-tunnel investigation was undertaken to evaluate the ability of a small leading-edge flap attached to the pressure surface of a Clark Y airfoil to improve low-Reynolds-number performance. Different flap geometries and designs were evaluated. Testing encompassed the measurement of lift and drag, surface pressures, and surface-flow topology rendered using titanium dioxide. Tests were undertaken at Reynolds numbers of 75,000 and 150,000. The results show that the flaps shift the zero-lift angle of attack in the positive direction, with the lift decrement stemming from reduced lower-surface pressure. The flaps proved to be beneficial only at high angles of attack, where they cause a sustained lift plateau and eliminate hysteresis.

Experimental Investigation of Double-Hinged Vortex Flap Configurations

D ELTA wings are poor lift generators. An often-employed design approach to ameliorate the low lift production is to sharpen thewing’s leading edge. A consequence of the enforced flow separation is the formation of leading-edge vortices. The vortices induce high surface velocities over the wing and may significantly augment lift, especially as the delta becomes more slender [1]. Leading-edge flaps (formed by downward rotation of a segment of the wing’s leading edge) may be used to lessen the performance loss by recovering thrust from the vortices by positioning them over wing elements with a forward-facing area: so-called leading-edge vortex flaps (LEVFs) [2]. Studies of vortex flaps (VFs) have shown the concept to be effective [2,3]; however, their performance is constrained by the characteristics of the leading-edge vortices with incidence. The vortices expand in size and migrate inboard off the flap. As a result, the flaps lose effectiveness. A study by Rinoie and Stollery [3] showed that the maximum lift-to-drag ratio L=D for the 60 and 70 deg delta wings they tested with vortex flaps coincided with smooth flow onto the flap with no separation. As a result, increasing wing incidence requires larger flap deflections to maintain maximized L=D values. Deflecting the flap reduces lift for a given geometric angle of attack as the vortex strength is reduced, as well as to a lesser extent, the attached flow lift [4]. An additional byproduct of flap deflection is a moderate increase in the minimum-drag coefficient. As a result, the initialL=D is attenuated, butmay increase significantly beyond that of the sharp planar delta for moderate to high lift coefficients. An attempt to reduce the drag penalty as well as limit the expansion of thevortex off theflapswas documented byRao [5]. Various flap configurations were investigated, variants including constant length and segmented configurations. The segmented flaps generated two smaller distinct leading-edge vortices so as to minimize vortex spillage. Rao’s results suggested that generating multiple leading-edge vortices was effective in delaying vortex spillage and thus suction loss from the flap. Generally, experimental observation has indicated that the leading-edge vortices on a planar delta lie along a trajectory of approximately 70% of the local semispan [6]. It may be feasible to design a vortex flap that is located further inboard, such that the flap hingeline runs along a ray of approximately 65% semispan. To limit the flap size, a tab may be employed, where the outer wing surface is inclined so as to be parallel to the inner wing section (see Fig. 1). A small tab was evaluated by Hoffler and Rao [7]; some of their designs resulted in the tab having a rearward inclination such that vortex formation over the tab would contribute additional drag. Other tabbed configurations that were tested were not deemed as successful [7]. In this paper, so-called double-hinged vortex flaps (HVF) are evaluated, where the configuration consists of a planar inboard portion, a vortex flap and a planar outboard panel. HVFs may reduce vortex loss with incidence (as its hingeline orientation is closer to the natural vortex trajectory, i.e., the flap is located further inboard), the minimum-drag penalty and structural complication. The undeflected outboard panel of the HVF may also augment the strength of the primary vortex compared with a conventional LEVF, where the effective leading-edge incidence is diminished. The proposed design would mitigate against a movable flap; thewing configuration would be fixed. The basis of comparison of the flaps in this study is different from that of Hoffler and Rao [7]. In the present study the flap area is conserved for a given flap area ratio, and does not include the area of the outboard panel (or tab). The configuration and size of the outboard panels are also conceptually different from those in [7], where the tabs were approximately constant chord and were not an integral part of the wing design, but an add-on. A low-speed windtunnel investigation was conducted to evaluate the concept, characterized through force-balance, pressure measurement, surface flow visualization, and vortex burst trajectories.

Effects of Surface Flats on the Performance of a Remotely Piloted Aircraft

T HE characterization of the aerodynamic performance of airfoils has been the subject of focused study since the 1910s. An implicitly assumed requirement is that the model fidelity (experimental or computational) is such that it is an exact rendering of the proposed full-scale flight article. In many instances, however, the actual wing profile may not be representative of its design. Generally, this becomes a greater issue as the size and dimensions of the vehicle reduce. With the proliferation of small-scale unmanned aerial vehicles, such concerns have increased in relevance. Wing geometry deviationsmay be due to damage, poor assembly, or simplified structure and design. This is commonly seen in radio-controlled (R/C) aircraft. Weight and cost considerations result in aircraft composed largely of balsa and plywood with aMonoKote skin covering. Generally, the wing skin is not supported between the ribs or the leadingand trailing-edge spars. This naturally results in flats when the skin is attached and stressed. As a result, the airfoil design and that actually implemented on the aircraft may be significantly different. Although wing/airfoil contour issues or alterations due to effects such as battle damage [1,2] have been investigated, little information appears available on a systematic study of contour effects thatmay result fromR/C modeling techniques. Consequently, a low-speed wind-tunnel investigation has been undertaken to quantify the effect of surface flats on the behavior of a Clark-Y airfoil. Data presented include surface pressure, wake survey, integral lift and drag coefficients, and surface flow visualization.

Experimental Investigation of Annular Wing Aerodynamics

A low-speed wind-tunnel investigation was conducted to explore the behavior of annular (ring) wings. Effects of aspect ratio as well as gap were investigated. Ring wings using a low Reynolds number Eppler section and a NACA 0012 profile were manufactured and tested. Measurements were recorded using a six-component sting balance. Experimental and theoretical trend comparisons were effected using a vortex-lattice code. The experimental results indicate wing efficiency factors well above 1 are achievable. The effect of gap was to increase the wing lift-curve slope as well as efficiency. The large increases in aerodynamic efficiency were generally mitigated by the significant minimum drag coefficient. Pitching moment characteristics were unfavorable and were dominated by dissimilar stall behavior between the upper and lower wing sections.

Effects of Vortex Generators on an Airfoil at Low Reynolds Numbers

A low-speed wind-tunnel investigation is presented detailing the effects of vortex generators on an airfoil at low Reynolds numbers (80,000 and 160,000). Six different static vortex generator layouts were tested. In addition, an oscillatory (or active) vortex generator was designed and tested. Force balance measurements were recorded and interpreted with the aid of surface flow visualization. The data suggest that the static vortex generators function similarly to those at higher Reynolds numbers; increasing the maximum lift coefficient and increasing the stall angle. Different static vortex generator configurations appear preferable at the two tested Reynolds number ranges. The oscillating vortex generator did not appear effective in its present configuration.